1. Field of the Invention
The present invention relates to an apparatus and method for detecting the end point in time of a post treatment after an in-situ cleaning operation.
2. Description of the Related Art
During the manufacture of thin film transistors (TFTs) for liquid-crystal displays or semiconductor devices, a plasma enhanced chemical -apor deposition (PECVD) apparatus is used to manufacture thin films of amorphous silicon (a-Si), or silicon-containing compounds, such as silicon nitride, silicon oxide or silicon oxynitride. In the PECVD apparatus, the silicon-containing compound films and silicon-based polymer are deposited on electrodes and on the inner wall of a reactor other than a substrate. The thickness of the deposited silicon-containing compound film increases as the thin films are repeatedly formed. When the thickness of the deposited silicon-containing compound film reaches a certain film thickness, the deposited silicon-containing compound film peels off from the inner wall and the electrodes, and forms fine particulates. If the fine particulates land on a substrate surface, defects occur in the TFTs, resulting in a deterioration in the yield of manufactured electric devices. Therefore, in a conventional PECVD apparatus, a silicon-containing compound deposited on the electrodes and the inner wall must be removed before the silicon-containing compound deposited on the electrodes and the inner wall peels off.
Since, in the PECVD apparatus, thin films are formed under a reduced pressure in a reactor, it is preferable that the silicon-containing compound be removed after a short period of time and without exposing the interior of the reactor to atmospheric air. For this purpose, performing an "in-situ cleaning" operation is most effective. The in-situ cleaning is what is commonly called a plasma self-cleaning technique. In the plasma self-cleaning technique, silicon-containing compounds deposited on the electrodes and the inner wall are removed by using reactive chemical species, such as fluorine radicals, produced by an electric discharge plasma. The plasma self-cleaning technique is widely used in PECVD apparatus for large-scale production requiring a high yield and a high-uptime ratio.
When the interior of the reactor is in-situ cleaned by active chemical species, these species adhere onto the electrodes and the inner wall of the reactor at the same time the silicon-containing compound deposited on the electrodes and the inner wall is removed. The reactive chemical species then remain inside the reactor after the in-situ cleaning process. Problems occur if an attempt is made to manufacture another silicon-containing compound thin film in the reactor while the reactive chemical species remain. For example, one of the following can occur: the silicon-containing compound film is not formed on the substrate at all; the deposition rate of the silicon-containing compound film decreases; the characteristics of the silicon-containing compound film deteriorate Considerably; or the silicon-containing compound film characteristics before the cleaning process are not reproduced. Therefore, it is necessary to remove the remaining reactive chemical species from the reactor after the in-situ cleaning process.
Various methods have been considered to remove the remaining reactive chemical species. Particularly effective is a seasoning operation after the in-situ cleaning process is completed but before the formation of the thin film is restarted. In the seasoning operation, a silicon hydride gas or a mixture of silicon hydride gas and another gas, such as for example hydrogen, nitrogen, or ammonia gas, which are used to form the thin film, are introduced into the reactor. A plasma is then generated in the silicon hydrogen gas or the mixture gas in order to remove the remaining reactive chemical species. This is hereinafter referred to as a "post treatment." According to this post treatment, reactive chemical species remaining on the electrodes and the inner wall, for example fluorine radicals, are sealed up in a silicon-based film formed on the electrodes and the inner wall, or they are transformed into a volatile silicon fluoride and evacuated. Reactive chemical species, which remain on the electrodes and the inner wall after the in-situ cleaning process, then react heavily with chemical species produced by a thin film formation gas plasma, such as the mixture gas plasma of silicon hydride and hydrogen. Thus, it is possible to almost completely remove the reactive chemical species by such a post treatment.
There has, however, been no method of directly detecting the end point of the post treatment. Hitherto, the end point has been determined empirically through data stored from the post treatment experiments in individual apparatus.
However, this presents a problem that when the end point is determined empirically, there is no confirmation as to whether the post treatment has been carried out inside the reactor for an appropriate processing time period. An excessive processing tire period causes the throughput to decrease and causes the electrodes and the inner wall cleaned in the in-situ cleaning process to be contaminated again at this stage of the post treatment. On the other hand, if there is an insufficient processing time period, reactive chemical species remaining after in-situ cleaning are nut completely removed. As a result, the thin film formed after the post treatment does not reproduce the thin film characteristics, such as electrical conductivity, achieved before the in-situ cleaning process.